It is well known as a conventional method for press-molding optical elements that optical elements can be obtained by setting preforms for molding optical elements, which have been manufactured previously by injection molding, into the space surrounded by an upper mold, a lower mold and a drum mold, and by molding the preforms under heat and pressure, as disclosed, for example, in Japanese laid-open patent application (Tokkai Hei) 8-127077. In this case, as shown in FIG. 10(a), a preform 64 for molding an optical element can be obtained via a gate 61, a runner 62 and a sprue 63. In case of obtaining optical elements by molding the preform 64 under heat and pressure, the preform 64 is cut off from the gate 61, which is used as a single substance, since the preform 64 can be obtained via the gate 61.
It is also well known as a conventional method for press-molding optical elements that optical elements can be obtained by softening a flat member to be processed with heat generated by vibration, and by forcing the member having a molding surface into the softened flat member, as disclosed, for example, in Japanese laid-open patent application (Tokkai Hei) 1-67309. A molding apparatus for carrying out this molding method will be explained as follows. As shown in FIG. 11, an upper die 52 and a lower die 53 are arranged facing each other and are supported by members not shown in the Figure, so that a flat member 54 to be processed, made from resin, can be supported by the upper die 52 and the lower die 53. A through hole 52a is formed along the central axis of the upper die 52, and an upper punch 55 can freely slide up and down in the through hole 52a. A through hole 53a is formed along the central axis of the lower die 53, and a lower punch 56 can freely slide up and down in the through hole 53a. On the end portion of the upper punch 55 and the lower punch 56, molding surfaces for molding optical elements are formed respectively. The lower punch 56 is fixed to a lower frame 57a of a frame 57, through which vibration is provided, and is also connected to an actuator 60 providing vibration, which is installed on the under surface of the lower frame 57a. A through hole 59 is provided in an upper frame 57b of the frame 57. The upper punch 55 is connected to a molding actuator 58 installed on the upper surface of the upper frame 57b, which can freely slide up and down in the through hole 59.
The following explanation is for the method for press molding optical elements using a molding apparatus having such a structure as mentioned above. First, a flat member 54 to be processed is supported by the upper die 52 and the lower die 53 and further the member 54 is firmly supported by shifting the upper punch 55 and the lower punch 56 axially. Next, the actuator 60 providing vibration is actuated and the upper punch 55 and the lower punch 56 are vibrated upwards and downwards by means of the frame 57 through which vibration is provided. As a result, the member 54 is softened with heat generated by self-heating. Then the upper punch is forced into the member 54 by actuating the molding actuator 58 while the actuator 60 providing vibration is operating. Consequently, the shape of the molding surface (the shape of an optical element) formed on the end portion of the upper punch 55 and the lower punch 56 is transferred onto the member 54. Finally, only the part on which the shape of an optical element is transferred is punched from the member 54 by shifting the upper punch 55 and the lower, punch 56 in the same direction, i.e. upwards or downwards. Accordingly, an optical element can be obtained.
However, in the preforms for molding optical elements, which are obtained by injection molding, the resin density near gates is ununiform compared to that of the other part and residual distortion near the gates is considerably high. Therefore, as shown in FIG. 10(b), the rise of temperature during the molding causes a recess 64a near the gate 61. As a result, the recessed part around the gate can get into the clear aperture of optical element 65 as shown in FIG. 12, and thereby a defect in the performance of optical elements might occur.
In the method for manufacturing optical elements by softening a flat member to be processed with heat generated by vibration, and by forcing the member for molding having a molding surface into the softened flat member, a closed space is created between the flat member 54 to be processed and the molding surfaces (recess) 55a and 56a formed on the end portion of the upper punch 55 and the lower punch 56, as shown in FIG. 13(a). As a result, a spatial portion 71 (on which the shape of optical element has not been transferred) might remain on the spherical portion 65a of an optical element 65 as shown in FIG. 13(b), thereby resulting in a defective product. Furthermore, a defect in the performance of optical elements might occur, since the shape of the molding surfaces (recess) 55a and 56a of the upper punch 55 and the lower punch 56 respectively is different, and thereby the precision of the shape transfer at the time of cutting is also different, or since the deviation of the axis of the upper punch 55 and the lower punch 56 causes the deviation of an optical axis in the upper and lower side of the spherical portion of optical elements.